The present invention generally relates to superconducting transistors, and more particularly to a superconducting transistor having a collector and an emitter made of a superconductor.
Recently, in computers and communication equipments, there are demands to carry out signal processing at higher speeds. Hence, much attention has been made on superconducting devices such as a Josephson device. In addition, various superconducting transistors have been proposed to replace the Josephson device.
Among previously proposed superconducting transistors, there is a superconducting transistor having a base made of a superconductor.
For example, in a superconducting transistor proposed in a Japanese Laid-Open Patent Application No.60-68681, an emitter, a base and a collector are all made of a superconductor. An insulator is provided as a barrier between the emitter and the base and between the base and the collector. In other words, this previously proposed superconducting transistor has the so-called SISIS structure, where the "S" stands for superconductor" and "I" stands for insulator.
On the other hand, a superconducting transistor proposed in a Japanese Laid-Open Patent Application No.60-10792 has an emitter and a base which are made of a superconductor. A collector is made of a normal conductor. In addition, an insulator is provided as a barrier between the emitter and the base and between the base and the collector. Hence, this previously proposed superconducting transistor has the so-called SISIN structure, where the "S" stands for "superconductor", "I" stands for "insulator" and "N" stands for "normal conductor".
According to superconducting transistor having the SISIS structure, the base is made of the superconductor and the Josephson junction is provided in duplicate. For this reason, a current-voltage characteristic of this superconducting transistor has a hysteresis curve for reasons similar to those in the case of the Josephson device. Accordingly, in order to return the state of this superconducting transistor from the voltage state to the superconducting state, the current must be decreased to zero, and there is a problem in that it is difficult to operate this superconducting transistor by a D.C. power supply. In addition, because the base is made of the superconductor and the energy band in the base region has no inclination, the reflection of electrons at the base-collector barrier is notable, and there is a problem in that this notable reflection introduces undesirable effects on the operation of the superconducting transistor.
On the other hand, according to the superconducting transistor having the SISIN structure, the collector is made of the normal conductor. Hence, even though the wiring with respect to the emitter can be formed by a superconductor, there is a problem in that a satisfactory contact cannot be obtained between the superconductor and the normal conductor if the wiring with respect to the collector is formed by the superconductor. On the other hand, if the collector is made thin in order to improve the operation speed of the superconducting transistor, there is a problem in that the collector, although made of the normal conductor, acts as a superconductor due to the proximity effect of the superconductor which forms the base if the collector is made of a metal. Further, if the normal conductor forming the collector is a semiconductor, the Schottky barrier formed between the base and the collector causes a problem. If the collector acts as the superconductor, the problems of the above SISIS structure are introduced. Moreover, if the Schottky barrier is formed between the base and the collector, there is a problem in that the signal amplifying function of the superconducting transistor deteriorates.